1. Field of the Invention
The present invention relates to an imaging apparatus and a method of driving the same.
2. Description of the Related Art
Digital cameras using a CMOS image sensor as an imaging device are known. An imaging device that applies analog to digital (AD) conversion to a pixel signal is known enabling signal readout at high speed. One of the AD conversion techniques has been known in which a comparator compares a pixel signal with a time-dependent reference signal (ramp signal) and acquires an AD-converted data according to the signal amplitude. The imaging device thus including an AD converter is expected to achieve high speed reading and high resolution.
In consideration of optical shot noise of the pixel signal, only bits enough for achieving the SN ratio are necessary. According to classification into multiple signal levels, high speed readout and high resolution can be achieved by reducing the number of bits. Further, a method of combining comparators and reference signals according to signal amplitudes has been known (e.g., see Japanese Patent Application Laid-Open No. 2007-281987).
The technique of Japanese Patent Application Laid-Open No. 2007-281987 utilizes a plurality of comparators. Accordingly, there is a problem in that the response speeds are different according to variation in manufacturing elements configuring the comparators, causing an error in the AD-converted data. Furthermore, there is difficulty in increasing a circuit area and power consumption.